A lot of things that we cannot buy and sell in markets used to be totally legal objects of market exchange - human beings when we had slavery, child labour, human organs, and so on. So there is no economic theory that actually says that you shouldn't have slavery or child labour because all these are political, ethical judgments.

There is no greater beast than envy, no greater thief than fear, no greater enemy than greed, no greater predator than wrath, and no greater poison than bitterness.